Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a hard storage disk that spins at a standard rotational speed. An actuator moves a magnetic read/write head over the disk. The actuator arm carries a head gimbal assembly (HGA) that includes a slider and a suspension with a nose portion for directly contacting a ramp used during the load and unload cycles for a load/unload drive. The slider carries a head assembly that includes a magnetic read/write transducer or head for reading/writing information to or from any desired location on the disk.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Areal densities of hard disk drives (HDD) in the past have increased at significant rates of 60 percent to more than 100 percent per year. This trend has slowed more recently to approximately 40 percent per year due to technology challenges. Areal densities today are close to 100 Gb/in2. HDDs are being used more often as digital applications in the consumer electronics industry proliferates, requiring much higher capacities and setting new expectation for lower acoustics. All of the above makes fluid dynamic bearing spindle motors attractive for minimizing non repeatable run-out (NRRO), lowering acoustical noise, and improving reliability.
Presently, ball bearing (BB) spindle motors comprise a significant number of shipments in hard disk drives. However, a transition to fluid dynamic bearings (FDB) is occurring in the HDD industry. The trend of incorporating FDB motors in HDD designs is a direct result of higher areal densities and much faster spindle speeds being achieved for today's applications. NRRO is the highest contributor to track mis-registration (TMR), thus impacting HDD performance. NRRO is also an inhibitor in achieving higher track densities. Ball bearing motors produce larger NRRO due to the mechanical contact with the inherent defects found in the geometry of the race ball interface and the lubricant film. Ball bearing spindle motors have minimized this issue with tighter tolerances and closer inspections. There is an upper limit beyond which the ball bearing design can no longer overcome the NRRO problem at the higher areal densities. Currently with ball bearings, NRRO has settled in the 0.1 micro-inch range.
By contrast, FDBs generate less NRRO due to absence of contact between the sleeve and stator. FDB designs are expected to limit NRRO in the range of 0.01 micro-inch. Other inherent properties of the FDB design are higher damping, reduced resonance, better non-operational shock resistance, greater speed control, and improved acoustics. Non-operational shock improvement is a result of a much larger area of surface-to-surface contact. Noise levels are reduced to approximately 20 dBA, since there is no contributing noise from ball bearings.
However, one problem with FDB is the loss of the lubrication or fluid within the bearing. As fluid is lost, the remaining fluid in the FDB bearing will retain more heat, have a larger concentration of sediment, provide reduced friction protection and generally impair FDB operation. Moreover, the loss of fluid enhances the problem resulting in additional fluid loss due to the heat build up induced evaporation, while the increased sediment concentration will increase wear. In other words, once the fluid loss begins, the time to catastrophic failure is significantly reduced.
Presently, the FDB can lose a significant amount of fluid from the capillary buffer via evaporation and atomization of the fluid as well as other operational factors. The loss of the fluid at the FDB will result in initial errors based on increased NRRO and finally seizure of the FDB.